PolylineExtensions.GetGeometry for 3DPolylines

no I would recommend to clean the DWG

The OOTB node does not create a good PolyCurve from a Polyline3D. A bug I guess?
Here is a workaround: Explode the Polyline3D into Lines. And then create a PolyCurve from the Lines.
(Don’t forget to Erase the Lines.and recreate a Polyline3D from the PolyCurve)

What is your definition of a “good PolyCurve”? It may look distorted/jagged in Dynamo, but that’s probably because it is really far from the origin. If you get the PolyCurve from C3D and then output a new 3D Polyline back into model space, it should be identical to the original.

Strange. I ran the same script at another PC system. No issues with converting the 3D polyline to a PolyCurve.
I think it has to do with the problem of not able to create a solid object from a DS solid to AutoCAD.